The present invention is related to fluid-pressure-operated, empty/load brake control apparatus for a railway freight car, and particularly for a railway freight car having short travel springs on which the car is supported.
Since the car springs become compressed with increasing loads, the degree of spring deflection serves as an indication of the car load condition, i.e., whether the car is in a "full" or "empty" load condition. Generally, a sensing arm or the like is arranged between the sprung and unsprung portions of the car, as a means of detecting the degree of spring deflection and thus the car load condition.
Generally, older vintage freight cars employ short travel type springs and are equipped with the old standard AB-7 5/8-12 Automatic Empty and Load Freight Brake Equipment, as described in WABCO Instruction Pamphlet No. 5062-5. Briefly, this equipment is characterized by a strut cylinder that is mounted on the end of the bolster (sprung portion) and includes a strut piston having a foot which is aligned to engage a stop on the truck side frame (unsprung portion). The piston is actuated by a cut-off valve to detect the degree of spring deflection in order to set a changeover valve to the appropriate load setting only during periods when the car brake pipe is being recharged, as when the car is picked up from a siding where it had been set out for loading or unloading. In this manner, the empty/load condition of the freight car is determined by operation of the strut cylinder for each load change and the strut cylinder is thereafter cut out in order to isolate the effect of car rock and roll during over-the-road service and thus prevents the consequent variation in the degree of spring deflection from influencing the load setting. Because of the complexity of this equipment, it is relatively costly and expensive to maintain.
Presently, there is an effort underway to upgrade these older vintage cars with the newer standard SC-1 Empty/Load Brake Equipment shown in U.S. Pat. No. 3,671,086 and issued June 20, 1972 to the assignee of the present invention. This equipment is characterized by a proportional valve via which fluid under pressure is connected from the car control valve device to the brake cylinder and a single load sensor valve that detects the car load condition according to the deflection of the car springs. In the "full" load condition, the fluid pressure developed by the car control valve is delivered directly through the proportional valve to the car brake cylinder, but in the "empty" load condition, the load sensor valve connects the brake cylinder delivery pressure to the proportional valve to cause the proportional valve to reduce the fluid pressure delivered to the brake cylinder by a predetermined percentage.
The single load sensor valve is mounted at one side of the car body (sprung portion) above the truck side frame (unsprung portion) and includes a sensor arm that is actuated during the time a brake application is in effect to sense the relationship of the car body to the side frame, as an indication of spring deflection and thus the car load condition. It will be appreciated, therefore, that contrary to the aforementioned AB-7 5/8-12 empty/load equipment used on older vintage cars, the SC-1 empty/load equipment is continuously effective during a brake application and is thus simpler in construction and accordingly less costly in both initial cost and to maintain. In order to prevent car rock or roll from affecting the load setting of this continuous acting type sensor valve, a simple lost-motion arrangement is provided between the sensor arm and valving to prevent a false changeover or transition between the "full" and "empty" load settings due to car rock and roll.
In attempting to adapt this less costly SC-1 empty/load equipment on cars having short travel springs, however, it is probable that due to the effect of car rock and roll combined with the normal car bolster side bearing clearance, sensor arm movement may exceed the available lost-motion, and could result in a false sensor reading of the car load condition. It will be appreciated that such false sensor readings result in undesirable brake pressures, due to cycling of the sensor valve between the "full" and "empty" load settings. In the case of a loaded car, this cycling of the load sensor valve is undesirable from the standpoint that reduced braking results when the load sensor valve cycles to the "empty" load setting. Such reduced braking, of course, can result in longer stopping distances.